This section provides background information related to the present disclosure, which is not necessarily prior art.
To determine fault direction, directional elements use the concept of torque. In digital relays the torque is computed with the current and voltage phasors, which are in turn calculated using discrete Fourier transform from the output signals of the current and potential transformers of the relay. There are six basic torques that are commonly used—a phase torque for each of the three phases, as well as the positive, negative, and zero sequence torques.
Directional elements are needed within electric power distribution systems to differentiate between load current and fault current when two or more sources are present in the distribution network. When a fault current is about two- or three-times the load current, this is easy to accomplish. But when the directional element is used in a relay that is protecting a distributed generation source, much lower fault current may need to be detected to identify the fault direction correctly. Moreover, certain limitations are recognized for individual phase torques with a particular torque angle, and of the positive sequence torque in general, particularly in distributed generation systems.
Accordingly, it is desirable to provide fault detection and fault direction determination with a phase overcurrent relay using individual phase torques and the negative sequence torque, and with a ground overcurrent relay using the zero sequence torque. It is also desirable to provide a device utilizing minimum pickup thresholds that will assure correct operation of the directional elements. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.
Corresponding reference numerals indicate corresponding parts throughout the several views of the drawings.